EUV collector systems are used in EUV lithography systems to collect EUV radiation from an EUV radiation source and direct the EUV radiation to an aperture typically referred to as or associated with the intermediate focus. The radiation from the intermediate focus is then relayed by an illuminator to illuminate a reflective reticle. Radiation reflected from the illuminated reticle is then projected onto a wafer coated with a photosensitive material such as photoresist that records the reticle image. The wafer is then processed to form integrated microcircuits.
FIG. 1 is a schematic diagram of a generalized configuration of a collector system 10N that uses a normal-incidence collector (NIC) mirror MN. FIG. 2 is a schematic diagram of a generalized configuration of a collector system 10G that uses a grazing-incident collector (GIC) mirror MG. Each collector system 10N and 10G has an EUV radiation source RS that emits EUV radiation 12, a central axis A1, and an intermediate focus IF. Each collector system 10N and 10G is shown arranged adjacent an illuminator 20 that has an entrance aperture member 22 that defines an entrance aperture 24. Entrance aperture member 22 is arranged at or near the intermediate focus IF. NIC mirror MN has a common input and output side 17, while GIC mirror MG has an input end 16 and an output end 18.
In each collector system 10N and 10G, an important performance metric for EUV lithography is the amount and angular distribution of EUV radiation 12 the collector mirror MN or MG can deliver to the intermediate focus IF and through the entrance aperture 24 of illuminator 20. As mentioned above, also of importance is the angular distribution of the EUV radiation 12 delivered through entrance aperture 24 of illuminator 20. Entrance aperture 24 is used to define the limits of the intermediate focus IF so that illuminator 20 can have the proper field size and numerical aperture for illuminating the reticle (not shown).
However, because neither type of collector system 10N or 10G can be made to perform perfectly, and because of magnification constraints on the system design, entrance aperture member 22 of illuminator 20 may also end up intercepting a substantial amount of EUV radiation 12L, so that this intercepted EUV radiation 12L is lost and is not utilized by the illuminator 20, as illustrated in FIG. 3.
Also, due to design limitations or manufacturing imperfections in the collector system 10N or 10G, EUV radiation 12 passing through the entrance aperture 24 may not have the optimum angular distribution for use by the illuminator 20. This lost or non-optimum EUV radiation 12L is problematic because as much useable EUV radiation 12 as possible must be provided to illuminator 20 so that there is sufficient radiation to uniformly illuminate the reticle and adequately expose the photosensitive material (photoresist) on the wafer.